The present invention relates to semiconductor devices, and in particular, to a semiconductor device with a conductivity modulation effect which is used as a semiconductor switching element in power converters and power controllers.
Most solid-state devices are inherently unilateral since they are designed to carry current in only one direction, known as the forward direction. Appreciable current flow in the reverse direction is blocked, although a brief transient of reverse current flows when reverse voltage is applied. Most solid-state switches open automatically when reverse polarity is applied across their terminals.
Switch devices come in three types. The first type is a two-terminal switch which automatically doses when forward polarity is applied and opens when reverse polarity is applied. Such devices are known as rectifier diodes or diodes. The second type of switch has an added control terminal. This type of switch blocks forward current until a turn-on pulse is applied to the control terminal. This switch continues to conduct as long as forward current flows. The control electrode cannot interrupt the forward current flow. Such devices are known as reverse-blocking triode thyristors, thyristors, or silicon controlled rectifiers (SCR's).
The third type of switch is similar to the second type except that it includes turnoff ability. That is, forward current can be interrupted by the control electrode. This type includes switching transistors because collector current ceased when the control signal is removed from the base. Bistable switches of this type are known as turnoff thyristors or gate-controlled switches. The present invention relates to this third type of switch.
Power semiconductor devices, which mainly carry out switching operations in power converters and power controllers, must have a small turn-on voltage drop in order to keep power loss to a minimum. Thyristors with a conductivity modulation effect and insulated gate bipolar transistors (hereinafter abbreviated to IGBT) are commonly used in applications that require a high blocking voltage. Conductivity modulation is defined as the variation of the conductivity of a semiconductor by varying the charge-carrier density.
The lateral structure shown in the prior art of FIG. 2 is suitable for incorporating a plurality of IGBT's in an integrated circuit. The IGBT's are built into a surface layer of an epitaxial n.sup.- layer 2 with a high-resistance factor grown on a p.sup.- silicon substrate 1. The IGBT's are formed on a chip 10 in the integrated circuit by producing more than two unit constructions U.sub.h as shown in FIG. 2, and connecting units U.sub.h in parallel. This unit construction U.sub.h is repeated symmetrically in the horizontal direction in FIG. 2. On the left side of FIG. 2, a p-type base region 3 is formed by diffusing impurities from a surface of n.sup.- layer 2, upon which an n.sup.+ emitter layer 4 is formed sandwiching a p.sup.+ contract region 31. A first gate electrode 51 on a gate oxide film 52 is connected to a gate terminal G. Gate terminal G is on a surface of a part of p-type base region 3 that is between n.sup.- layer 2 and n.sup.+ emitter region 4. In addition, an emitter electrode 61 connected to a first main terminal T1 contacts both emitter region 4 and contact region 31. On the right side of FIG. 2, a p.sup.+ drain region 7 surrounded by an n-type buffer region 21 is formed by diffusing impurities from the surface of n.sup.- layer 2 in the same manner as described above. A collector electrode 62 connected to a second main terminal T2 contacts drain region 7.
In this horizontal type IGBT, p-type base region 3, n.sup.- layer 2 and p.sup.+ drain region 7 constitute one PNP transistor. This bipolar transistor is turned on and off by controlling the potential of first gate electrode 51 that injects a base current into n.sup.- layer 2, which acts as the base of the transistor. That is, when positive voltage is applied from gate terminal G into gate 51, a plurality of electrons e, the majority carrier, flow into n.sup.- layer 2 from emitter region 4 via an n-type inversion layer (not shown) formed on a surface of p-type base region 3 directly below gate 51, thereby creating conduction between main terminals T1 and T2 of the IGBT.
Furthermore, because a plurality of holes h, the minority carrier, are injected into n.sup.- layer 2 from p.sup.+ drain region 7 via buffer region 21 by the base current after the conduction between main terminals T1 and T2 is created, the conductivity modulating action reduces the on-voltage between main terminals T1 and T2. The on-voltage is thus lower than in an ordinary MOSFET. This IGBT may be turned off by stopping the injection of majority carrier e into n.sup.- layer 2 by turning the voltage at gate 51 off. A depletion layer extends to n.sup.- layer 2 after the device is turned off to create a high reverse voltage blocking condition.
A horizontal MOS controlled thyristor (hereinafter referred to as the MCT) according to the prior art, shown in FIG. 3, is controlled by a MOS gate. A p.sup.+ emitter region 41 is formed on a surface of an n-type base region 32, which itself is on a surface of a p-type base region 3. An emitter electrode 61 contacts both p.sup.+ emitter region 41 and n-type base layer 32. A gate 51, on a gate oxide film 52, is disposed on the surface of n-type base 32 and p-type base 3 in a region sandwiched between n.sup.- layer 2 and p.sup.+ emitter region 41. When a positive voltage is applied from gate terminal G into gate 51, electrons (not shown) flow into n.sup.- layer 2 from n-type base region 32 via an n-type inversion layer (not shown) formed on a surface of p-type base region 3 directly below gate 51. Conduction is thereby created between main terminals T1 and T2, causing conductivity modulation as in the case of the IGBT. When the thyristor is turned off, a negative voltage is applied to gate 51. As a result, an n-channel on the surface of p-type base region 3 is closed while a p-channel is formed on the surface of n-type base region 32 directly below gate 51. Therefore, p-type base region 3 is shorted with emitter electrode 61 via p.sup.+ emitter region 41, and the thyristor is turned off.
As described above, the IGBT has an advantage in that it is turned on and off and controlled easily through insulating gate 51 which has a high input impedance. When the IGBT is on, the on-voltage is reduced by the conductivity modulation effect in n.sup.- layer 2. However, the excessive carriers that contribute to conductivity modulation when the device is on must be swept away to expand the depletion layer when the device is turned off. This causes a problem because a lot of time is consumed in removing the carriers, substantially prolonging the turn-off time and increasing the turn-off loss. This increase in turn-off loss is a major obstacle to using the IGBT in a high-frequency circuit requiting high-speed on/off operations.
The turn-off loss noticeably increases when driving an inductive load, which generates a counter electromotive force following extension of the depletion layer during turn-off. The extension of the depletion layer causes a sweeping out of majority carriers that acts as a base current of the PNP transistor and leads to an injection of the minority carriers to maintain the current for an inductive load. The constant current and large counter electromotive force simultaneously applied to the device increases the turn-off loss. To improve such turn-off characteristics, minority carriers injected from drain region 7 into n.sup.- layer 2 during a turn-on operation may be reduced by raising the concentration of impurities in buffer region 21. However, this action has its limits because it has a negative effect on the conductivity modulation during operation in the on-state. It is also possible to shorten the carrier life time to accelerate recombination of the carriers by diffusing heavy metals such as gold and platinum into the region, since these are so-called carrier life time killers. However, this technique has the negative effect of increasing the on-voltage. The problems described above occur in both the IGBT and the MCT.